Riverbed rise threatens Franz Josef settlement
Hydrologists warn the Waiho River is filling vertically behind stopbanks, leaving the town one major flood away from major structural failure.
By Matthew Qureshi ·
Ian Fuller, a hydrologist on the Waiho Technical Advisory Group, told West Coast Regional councillors that the Waiho River is one flood away from a disaster. The riverbed in front of the settlement’s stopbanks has risen two metres above the ground level of Franz Josef. The rise is accelerating as gravel continues to pile up behind the barriers.
Fuller stated the riverbed has been climbing 20 centimetres per year. He attributed this rate to gravel buildup caused by the retreating Franz glacier. While natural processes drive the rise, Fuller argued that the network of historic and newer stopbanks has worsened the situation. These structures, built in the 1980s, have confined the river to one-third of its natural area.
“Rather than being able to spread the load across a wide area, that cannot be spread anymore,” Fuller said. “So rather than spreading naturally, it is building vertically.” The confined channel forces sediment to stack upward instead of spreading out, raising the floor of the river rapidly.
The risk is compounded by millions of cubic metres of sediment trapped in the upper valley above the State Highway 6 bridge. Fuller noted that this material has been waiting for three years to move. He expects large storms to remobilise this sediment, sending it downstream toward the township.
The Waiho Technical Advisory Group released six recommendations last week. The group warned that the stopbank system has effectively restricted the river for the past 30 years. This restriction increases the risk to residents living behind the banks. The group also cited a 15 percent chance of a magnitude 8 Alpine Fault earthquake in the next 10 years.
Such an earthquake would likely cause severe damage to the stopbanks. It would trigger accelerated aggradation and flooding from land sliding in the catchment. Fuller described the overdue earthquake as a real factor that makes addressing the issue more urgent. The 2023 strategy predicted that increasing flood risk would make the stopbanks more vulnerable to overtopping or breaching.
The advisory group’s primary recommendation is to remove the stopbanks on the south bank. This would allow the river to fan out across the Waiho Flat farming area south of the settlement. Fuller said this would create a more resilient response to large storm events. “The river will be better set to be able to handle that supply of sediment,” he said.
Westland mayor Helen Lash announced the formation of a committee to create a masterplan for the Franz Josef area. The committee aims to have a formal proposal ready for ministers early next year. The government requires this plan to consider future funding applications for shifting infrastructure.
